1. 在 Form1 中加入两个 CommandButton 和一个 PictureBox.

2. 在 Form1 的代码窗口中添加以下代码：
Private Type RECT
Left As Long
Top As Long
Right As Long
Bottom As Long
End Type

Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function ReleaseDC Lib "user32" (ByVal hwnd As Long, ByVal hdc As Long) As Long
Private Declare Function CreatePatternBrush Lib "gdi32" (ByVal hBitmap As Long) As Long
Private Declare Function PatBlt Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal dwRop As Long) As Long
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long
Private Declare Function CreateBitmap Lib "gdi32" (ByVal nWidth As Long, ByVal nHeight As Long, ByVal nPlanes As Long, ByVal nBitCount As Long, lpBits As Any) As Long
Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long
Private Declare Function InvalidateRect Lib "user32" (ByVal hwnd As Long, ByVal lpRect As Long, ByVal bErase As Long) As Long
Private bybits(1 To 16) As Byte
Private hBitmap As Long, hBrush As Long
Private hDesktopWnd As Long

Private Sub Command1_Click()
Dim rop As Long, res As Long
Dim hdc5 As Long, width5 As Long, height5 As Long
hdc5 = GetDC(0)
width5 = Screen.Width / Screen.TwipsPerPixelX
height5 = Screen.Height / Screen.TwipsPerPixelY
rop = &HA000C9
Call SelectObject(hdc5, hBrush)
res = PatBlt(hdc5, 0, 0, width5, height5, rop)
Call DeleteObject(hBrush)
res = ReleaseDC(0, hdc5)
End Sub

Private Sub Command2_Click()
Dim aa As Long
aa = InvalidateRect(0, 0, 1)
End Sub
Private Sub Form_Load()
Dim ary
Dim i As Long
ary = Array(&H55, &H0, &HAA, &H0, _
&H55, &H0, &HAA, &H0, _
&H55, &H0, &HAA, &H0, _
&H55, &H0, &HAA, &H0)
For i = 1 To 16
bybits(i) = ary(i - 1)
Next i
hBitmap = CreateBitmap(8, 8, 1, 1, bybits(1))
hBrush = CreatePatternBrush(hBitmap)
Picture1.ForeColor = RGB(0, 0, 0)
Picture1.BackColor = RGB(255, 255, 255)
Picture1.ScaleMode = 3
End Sub

运行程序，按 Command1 就可以使屏幕暗下来，按 Command2 恢复。